As described in Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, volume 17, pages 353-355, the term “plexifilamentary yarn” refers to a yarn or strand characterized by a morphology substantially consisting of a three-dimensional integral network of thin, ribbon-like, film-fibril elements of random length that have a mean film thickness of less than about 4 microns and a median fibril width of less than 25 microns, and that are generally coextensively aligned with the longitudinal axis of the yarn. In plexifilamentary yarns, the film-fibril elements intermittently unite and separate at irregular intervals in various places throughout the length, width and thickness of the yarn thereby forming the three-dimensional network.
A plexifilimentary yarn is produced by flash-spinning of polyethylene or other polymers. Flash spinning is well-known in the art as a process for preparing plexifilimentary yarns and the non-woven fabrics made therefrom, as well as microcellular foams. Whether a plexifilimentary yarn or a microcellular foam is obtained depends upon the particulars of the spinning process, as described below. In flash-spinning, a polymer, usually polyethylene, is dispersed in a non-solvent, known as a spin agent. The dispersion so formed is then subject to elevated temperature and pressure to form a homogeneous solution, and the solution is then fed to an orifice at lower pressure. At the orifice, the highly pressurized solvent phase separates and rapidly vaporizes, leaving behind a unique three dimensional structure which may be a plexifilimentary yarn or a microcellular foam.
The morphology of, and means for preparing, a plexifilimentary yarn and a microcellular foam are described in detail in U.S. Pat. No. 3,081,519, which is incorporated in its entirety as a part hereof. The use of plexifilimentary yarns in the formation of non-woven fabrics is described in U.S. Pat. No. 3,169,899 and U.S. Pat. No. 3,442,740. Plexifilamentary yarns have found widespread commercial value primarily in the form of flash-spun high density polyethylene non-woven fabrics, most notably Tyvek® non-woven fabric, which is manufactured by the DuPont Company.
There have been numerous reports in the art of efforts directed at achieving one or another modification of the properties of flash-spun polyethylene by employing other polymers, either alone or in combination with polyethylene. For example, U.S. Pat. No. 6,004,672 discloses flash-spinning of blends of polyethylene and polypropylene. In another example, U.S. Pat. No. 6,136,911 discloses flash-spinning of blends of partially fluorinated copolymers such as polyvinylidene fluoride or ethylene/tetrafluoroethylene with 12-30% by weight of polyethylene.
In another approach to achieving modified properties in flash spun polyethylene, GB 891,944 discloses radiation induced grafting of so-called organic modifiers on to the surface of plexifilimentary yarns and non-woven fabrics produced therefrom. Suitable organic modifiers include ethylenically unsaturated monomers, including fully and partially fluorinated monomers. Also included are unsaturated and saturated polymers. In U.S. Pat. No. 6,096,421, a maleic anhydride grafted polyethylene was used as a modifier for ethylene/vinyl alcohol copolymer.